The present invention generally is related to electrical machines, especially motors, and more precisely to the structure of the stator assembly thereof.
Electrical machines work by the interaction of magnetic flux and current. At operating limits the magnetic flux is limited by saturation of the soft magnetic material and the current by the temperature resulting from ohmic heating. If more space is provided for conductor to reduce current densities and hence ohmic heating, the magnetic flux is also reduced and vice versa if more soft magnetic material is included the flux is increased but the conductor area is reduced and ohmic heating increased.
In most machines the conductors are contained in slots. It is necessary to semi-close the slots to obtain a smooth torque output and the result is that placing the windings in the slots is a difficult task which results in a rather poor ratio of conductor area to slot area. Ratios in the region of 50% are considered good. This poor ratio is bad for two reasons, firstly space is wasted which could be used for conductor or soft magnetic material and secondly the space in the slot will act as a thermal barrier increasing the temperature for a given ohmic loss.
An object of this invention is to provide a stator core for an electrical machine which stator core provides a better ratio of conductor area to slot area than stators in the prior art.
This object is achieved in that a stator in accordance with the present invention is characterised by the features specified in the appended claim 1. Preferred embodiments of that stator are defined in the dependent claims.
Thus, this invention is concerned with exploiting the good surface finish, tight dimensional tolerance and three dimensional magnetic flux carrying capabilities of soft magnetic composites by breaking a motor""s iron core into separate tooth and core back sections. These separate sections allow separately produced simple bobbin wound type coils to be assembled with the core sections to make units which are joined together and assembled within the motor""s frame.
The coils being produced away from the geometric difficulties of insertion into the slots can have a far higher ratio of copper to slot area; using machine winding 70% is easily achieved. If the further step is taken to compress the coils in a die, 81% has been achieved which is getting close to the theoretical maximum (which is fixed by the need for insulation around the conductors).
The result of this high ratio of conductor to slot area is a major reduction in coil resistance and hence in ohmic losses and a large increase in thermal conduction. The result is that at limit the motor will produce far more output, lowering capital cost and offering efficiency, size and weight benefits.
At the same time, the sub-components proposed are easily produced by cheap and fully automated processes, and the requirements for mechanical strength, withstand of electrical breakdown and dimensional tolerance are easily met. This is in contrast to a conventional machine whose winding either requires much handwork or expensive and difficult-to-set-up winding machines.